Botany called plant science, plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist; the term "botany" comes from the Ancient Greek word βοτάνη meaning "pasture", "grass", or "fodder". Traditionally, botany has included the study of fungi and algae by mycologists and phycologists with the study of these three groups of organisms remaining within the sphere of interest of the International Botanical Congress. Nowadays, botanists study 410,000 species of land plants of which some 391,000 species are vascular plants, 20,000 are bryophytes. Botany originated in prehistory as herbalism with the efforts of early humans to identify – and cultivate – edible and poisonous plants, making it one of the oldest branches of science. Medieval physic gardens attached to monasteries, contained plants of medical importance, they were forerunners of the first botanical gardens attached to universities, founded from the 1540s onwards.
One of the earliest was the Padua botanical garden. These gardens facilitated the academic study of plants. Efforts to catalogue and describe their collections were the beginnings of plant taxonomy, led in 1753 to the binomial system of Carl Linnaeus that remains in use to this day. In the 19th and 20th centuries, new techniques were developed for the study of plants, including methods of optical microscopy and live cell imaging, electron microscopy, analysis of chromosome number, plant chemistry and the structure and function of enzymes and other proteins. In the last two decades of the 20th century, botanists exploited the techniques of molecular genetic analysis, including genomics and proteomics and DNA sequences to classify plants more accurately. Modern botany is a broad, multidisciplinary subject with inputs from most other areas of science and technology. Research topics include the study of plant structure and differentiation, reproduction and primary metabolism, chemical products, diseases, evolutionary relationships and plant taxonomy.
Dominant themes in 21st century plant science are molecular genetics and epigenetics, which are the mechanisms and control of gene expression during differentiation of plant cells and tissues. Botanical research has diverse applications in providing staple foods, materials such as timber, rubber and drugs, in modern horticulture and forestry, plant propagation and genetic modification, in the synthesis of chemicals and raw materials for construction and energy production, in environmental management, the maintenance of biodiversity. Botany originated as the study and use of plants for their medicinal properties. Many records of the Holocene period date early botanical knowledge as far back as 10,000 years ago; this early unrecorded knowledge of plants was discovered in ancient sites of human occupation within Tennessee, which make up much of the Cherokee land today. The early recorded history of botany includes many ancient writings and plant classifications. Examples of early botanical works have been found in ancient texts from India dating back to before 1100 BC, in archaic Avestan writings, in works from China before it was unified in 221 BC.
Modern botany traces its roots back to Ancient Greece to Theophrastus, a student of Aristotle who invented and described many of its principles and is regarded in the scientific community as the "Father of Botany". His major works, Enquiry into Plants and On the Causes of Plants, constitute the most important contributions to botanical science until the Middle Ages seventeen centuries later. Another work from Ancient Greece that made an early impact on botany is De Materia Medica, a five-volume encyclopedia about herbal medicine written in the middle of the first century by Greek physician and pharmacologist Pedanius Dioscorides. De Materia Medica was read for more than 1,500 years. Important contributions from the medieval Muslim world include Ibn Wahshiyya's Nabatean Agriculture, Abū Ḥanīfa Dīnawarī's the Book of Plants, Ibn Bassal's The Classification of Soils. In the early 13th century, Abu al-Abbas al-Nabati, Ibn al-Baitar wrote on botany in a systematic and scientific manner. In the mid-16th century, "botanical gardens" were founded in a number of Italian universities – the Padua botanical garden in 1545 is considered to be the first, still in its original location.
These gardens continued the practical value of earlier "physic gardens" associated with monasteries, in which plants were cultivated for medical use. They supported the growth of botany as an academic subject. Lectures were given about the plants grown in the gardens and their medical uses demonstrated. Botanical gardens came much to northern Europe. Throughout this period, botany remained subordinate to medicine. German physician Leonhart Fuchs was one of "the three German fathers of botany", along with theologian Otto Brunfels and physician Hieronymus Bock. Fuchs and Brunfels broke away from the tradition of copying earlier works to make original observations of their own. Bock created his own system of plant classification. Physician Valerius Cordus authored a botanically and pharmacologically important herbal Historia Plantarum in 1544 and a pharmacopoeia of lasting importance, the Dispensatorium
Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it transports nutrients; the word "xylem" is derived from the Greek word ξύλον, meaning "wood". The term was introduced by Carl Nägeli in 1858; the most distinctive xylem cells are the long tracheary elements. Tracheids and vessel elements are distinguished by their shape. Xylem contains two other cell types: parenchyma and fibers. Xylem can be found: in vascular bundles, present in non-woody plants and non-woody parts of woody plants in secondary xylem, laid down by a meristem called the vascular cambium in woody plants as part of a stelar arrangement not divided into bundles, as in many ferns. In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although a vascular bundle will contain primary xylem only; the branching pattern exhibited by xylem follows Murray's law.
Primary xylem is formed during primary growth from procambium. It includes metaxylem. Metaxylem develops before secondary xylem. Metaxylem has wider tracheids than protoxylem. Secondary xylem is formed during secondary growth from vascular cambium. Although secondary xylem is found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the two main groups in which secondary xylem can be found are: conifers: there are some six hundred species of conifers. All species have secondary xylem, uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood. Angiosperms: there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem is rare in the monocots. Many non-monocot angiosperms become trees, the secondary xylem of these is used and marketed as hardwood; the xylem and tracheids of the roots and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plants.
The system transports water and soluble mineral nutrients from the roots throughout the plant. It is used to replace water lost during transpiration and photosynthesis. Xylem sap consists of water and inorganic ions, although it can contain a number of organic chemicals as well; the transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees. Three phenomena cause xylem sap to flow: Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals; the phloem pressure can rise to several MPa, far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere creates a negative pressure at the top of a plant. This causes millions of minute menisci to form in the mesophyll cell wall; the resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil. Root pressure: If the water potential of the root cells is more negative than that of the soil due to high concentrations of solute, water can move by osmosis into the root from the soil; this causes a positive pressure. In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is highest in the morning before the stomata allow transpiration to begin. Different plant species can have different root pressures in a similar environment; the primary force that creates the capillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits.
Capillary action provides the force that establishes an equilibrium configuration, balancing gravity. When transpiration removes water at the top, the flow is needed to return to the equilibrium. Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves; this evaporation causes the surface of the water to recess into the pores of the cell wall. By capillary action, the water forms concave menisci inside the pores; the high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. Transpirational pull requires that the vessels transporting the water be small in diameter, and as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots the gases come out of solution and form a bubble – an embolism forms, which will spread to other adjacent cells, unless bordered pits are present (these have
The vascular cambium is the main growth layer in the stems and roots of many plants in dicots such as buttercups and oak trees, gymnosperms such as pine trees. It produces xylem on the phloem on the outside. In herbaceous plants, it occurs in the vascular bundles which are arranged like beads on a necklace forming an interrupted ring inside the stem. In woody plants, it grows new wood on the inside. Other names for the vascular cambium are wood cambium, or bifacial cambium. In more detail, the vascular cambium is a plant tissue located between the xylem and the phloem in the stems and roots of certain vascular plants, it is a cylinder of unspecialized meristem cells. It is the source of both secondary xylem growth inwards towards the pith, secondary phloem growth outwards to the bark. Unlike the xylem and phloem, it does not transport water, minerals or food through the plant. Vascular cambia are found in dicots and gymnosperms but not monocots, which lack secondary growth. A few leaf types have a vascular cambium.
In dicot and gymnosperm trees, the vascular cambium is the obvious line separating the bark and wood. For successful grafting, the vascular cambia of the rootstock and scion must be aligned so they can grow together; the cambium present between primary xylem and primary phloem is called the intrafascicular cambium. During secondary growth, cells of medullary rays, in a line between neighbouring vascular bundles, become meristematic and form new interfascicular cambium; the intrafascicular and interfascicular cambia thus join up to form a ring which separates the primary xylem and primary phloem, the cambium ring. The vascular cambium produces secondary xylem on the inside of the ring, secondary phloem on the outside, pushing the primary xylem and phloem apart; the vascular cambium consists of two types of cells: Fusiform initials Ray initials The vascular cambium is maintained by a network of interacting signal feedback loops. Both hormones and short peptides have been identified as information carriers in these systems.
While similar regulation occurs in other meristems of plants, the cambial meristem receives signals from both the xylem and phloem sides for the meristem. Signals received from outside the meristem act to down regulate internal factors, which promotes cell proliferation, promotes differentiation; the phytohormones that are involved in the vascular cambial activity are auxins, gibberellins, abscisic acid and more to be discovered. Each one of these plant hormones are vital for the regulation of the cambial activity and are dependent on their concentration. Auxin hormones are proven to stimulate mitoses, cell production and regulate interfascicular and fascicular cambium. Applying auxin to the surface of a tree stump allowed decapitated shoots to continue secondary growth; the absence of auxin hormones will have a detrimental effect on a plant. It has been shown that mutants without auxin will exhibit increased spacing between the interfascicular cambiums and reduced growth of the vascular bundles.
The mutant plant will therefore experience a decreased in water and photosynthates being transported throughout the plant leading to death. Auxin regulates the two types of cell in the vascular cambium and fusiform initials. Regulation of these initials ensures the connection and communication between xylem and phloem is maintained for the translocation of nourishment and sugars are safely being stored as an energy resource. Ethylene levels are high in plants with an active cambial zone and are still being studied. Gibberellin stimulates the cambial cell division and regulates differentiation of the xylem tissues, with no effect on the rate of phloem differentiation. Differentiation is an essential process that changes these tissues into a more specialized type, leading to an important role in maintaining the life form of a plant. In poplar trees, high concentrations of gibberellin is positively correlated to an increase of cambial cell division and an increase of auxin in the cambial stem cells.
Gibberellin is responsible for the expansion of xylem through a signal traveling from the shoot to the root. Cytokinin hormone is known to regulate the rate of the cell division instead of the direction of cell differentiation. A study demonstrated that the mutants are found to have a reduction in stem and root growth but the secondary vascular pattern of the vascular bundles were not affected with a treatment of cytokinin. Cambium Meristem Cork cambium Unifacial cambium Sun scald Pictures of Vascular cambium Detailed description - James D. Mauseth Review. "The vascular cambium: Molecular control of cellular structure". Protoplasma. 247: 145–161. Doi:10.1007/s00709-010-0211-z. PMID 20978810
Cork cambium is a tissue found in many vascular plants as part of the epidermis. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems, it is found in woody and many herbaceous dicots and some monocots. It is one of the plant's meristems – the series of tissues consisting of embryonic disk cells from which the plant grows, it is one of the many layers between the cork and primary phloem. The function of cork cambium is to produce a tough protective material. Synonyms for cork cambium are bark cambium and phellogen. Phellogen is defined as the meristematic cell layer responsible for the development of the periderm. Cells that grow inwards from there are termed phelloderm, cells that develop outwards are termed phellem or cork; the periderm thus consists of three different layers: phelloderm – inside of cork cambium. Commercial cork is derived from the bark of the cork oak. Cork has many uses including wine bottle stoppers, bulletin boards, hot pads to protect tables from hot pans, sealing for lids, gaskets for engines, fishing bobbers, handles for fishing rods and tennis rackets, etc.
It is a high strength-to-weight/cost ablative material for aerodynamic prototypes in wind tunnels, as well as satellite launch vehicle payload fairings, reentry surfaces, compression joints in thrust-vectored solid rocket motor nozzles. Many types of bark are used as mulch. Frost crack Sun scald
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines, shrubs. Bark is a nontechnical term, it consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm; the outer bark in older stems includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the last formed periderm is called the rhytidome. Products derived from bark include: bark shingle siding and wall coverings and other flavorings, tanbark for tannin, latex, poisons, various hallucinogenic chemicals and cork. Bark has been used to make cloth and ropes and used as a surface for paintings and map making. A number of plants are grown for their attractive or interesting bark colorations and surface textures or their bark is used as landscape mulch. What is called bark includes a number of different tissues.
Cork is an external, secondary tissue, impermeable to water and gases, is called the phellem. The cork is produced by the cork cambium, a layer of meristematically active cells which serve as a lateral meristem for the periderm; the cork cambium, called the phellogen, is only one cell layer thick and it divides periclinally to the outside producing cork. The phelloderm, not always present in all barks, is a layer of cells formed by and interior to the cork cambium. Together, the phellem and phelloderm constitute the periderm. Cork cell walls contain suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, prevents infections by bacteria and fungal spores; the cambium tissues, i.e. the cork cambium and the vascular cambium, are the only parts of a woody stem where cell division occurs. Phloem is a nutrient-conducting tissue composed of sieve tubes or sieve cells mixed with parenchyma and fibers; the cortex is the primary tissue of roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle.
From the outside to the inside of a mature woody stem, the layers include: Bark Periderm Cork, includes the rhytidome Cork cambium Phelloderm Cortex Phloem Vascular cambium Wood Sapwood Heartwood Pith In young stems, which lack what is called bark, the tissues are, from the outside to the inside: Epidermis, which may be replaced by periderm Cortex Primary and secondary phloem Vascular cambium Secondary and primary xylem. As the stem ages and grows, changes occur that transform the surface of the stem into the bark; the epidermis is a layer of cells that cover the plant body, including the stems, leaves and fruits, that protects the plant from the outside world. In old stems the epidermal layer and primary phloem become separated from the inner tissues by thicker formations of cork. Due to the thickening cork layer these cells die; this dead layer is the rough corky bark that forms around other stems. A secondary covering called the periderm forms on small woody stems and many non-woody plants, composed of cork, the cork cambium, the phelloderm.
The periderm forms from the phellogen. The periderm replaces the epidermis, acts as a protective covering like the epidermis. Mature phellem cells have suberin in their walls to protect the stem from desiccation and pathogen attack. Older phellem cells are dead; the skin on the potato tuber constitutes the cork of the periderm. In woody plants the epidermis of newly grown stems is replaced by the periderm in the year; as the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork. A limited number of cell layers may form interior to the cork cambium, called the phelloderm; as the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water and the cells outside the periderm, namely the epidermis and older secondary phloem die. Within the periderm are lenticels, which form during the production of the first periderm layer. Since there are living cells within the cambium layers that need to exchange gases during metabolism, these lenticels, because they have numerous intercellular spaces, allow gaseous exchange with the outside atmosphere.
As the bark develops, new lenticels are formed within the cracks of the cork layers. The rhytidome is the most familiar part of bark, being the outer layer that covers the trunks of trees, it is composed of dead cells and is produced by the formation of multiple layers of suberized periderm and phloem tissue. The rhytidome is well developed in older stems and roots of trees. In shrubs, older bark is exfoliated and thick rhytidome accumulates, it is thickest and most distinctive at the trunk or bole of the tree. Bark tissues make up by weight between 10–20% of woody vascular plants and consists of various biopolymers, lignin, suberin and polysaccharides. Up to 40% of the bark tissue is made of lignin which forms an important part of a plant providing stru
A meristem is the tissue in most plants containing undifferentiated cells, found in zones of the plant where growth can take place. Meristematic cells are responsible for growth. Differentiated plant cells cannot divide or produce cells of a different type. Meristematic cells are incompletely or not at all differentiated, are capable of continued cellular division. Therefore, cell division in the meristem is required to provide new cells for expansion and differentiation of tissues and initiation of new organs, providing the basic structure of the plant body. Furthermore, the cells are small and protoplasm fills the cell completely; the vacuoles are small. The cytoplasm does not contain differentiated plastids, although they are present in rudimentary form. Meristematic cells are packed together without intercellular cavities; the cell wall is a thin primary cell wall as well as some are thick in some plants. Maintenance of the cells requires a balance between two antagonistic processes: organ initiation and stem cell population renewal.
There are three types of meristematic tissues: apical and lateral. At the meristem summit, there is a small group of dividing cells, called the central zone. Cells of this zone are essential for meristem maintenance; the proliferation and growth rates at the meristem summit differ from those at the periphery. The term meristem was first used in 1858 by Carl Wilhelm von Nägeli in his book Beiträge zur Wissenschaftlichen Botanik, it is derived from the Greek word merizein, meaning to divide, in recognition of its inherent function. Apical meristems are the undifferentiated meristems in a plant; these differentiate into three kinds of primary meristems. The primary meristems in turn produce the two secondary meristem types; these secondary meristems are known as lateral meristems because they are involved in lateral growth. There are two types of apical meristem tissue: shoot apical meristem, which gives rise to organs like the leaves and flowers, root apical meristem, which provides the meristematic cells for future root growth.
SAM and RAM cells divide and are considered indeterminate, in that they do not possess any defined end status. In that sense, the meristematic cells are compared to the stem cells in animals, which have an analogous behavior and function; the number of layers varies according to plant type. In general the outermost layer is called the tunica. In monocots, the tunica determine the physical characteristics of the leaf margin. In dicots, layer two of the corpus determine the characteristics of the edge of the leaf; the corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the stem; some Arctic plants have an apical meristem in the lower/middle parts of the plant. It is thought. Shoot apical meristems are the source such as leaves and flowers. Cells at the shoot apical meristem summit serve as stem cells to the surrounding peripheral region, where they proliferate and are incorporated into differentiating leaf or flower primordia.
The shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, petals and ovaries are initiated here at the rate of one every time interval, called a plastochron, it is. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as auxiliary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome; the shoot apical meristem consists of 4 distinct cell groups: Stem cells The immediate daughter cells of the stem cells A subjacent organizing center Founder cells for organ initiation in surrounding regionsThe four distinct zones mentioned above are maintained by a complex signalling pathway. In Arabidopsis thaliana, 3 interacting CLAVATA genes are required to regulate the size of the stem cell reservoir in the shoot apical meristem by controlling the rate of cell division. CLV1 and CLV2 are predicted to form a receptor complex to. CLV3 shares some homology with the ESR proteins of maize, with a short 14 amino acid region being conserved between the proteins.
Proteins that contain these conserved regions have been grouped into the CLE family of proteins. CLV1 has been shown to interact with several cytoplasmic proteins that are most involved in downstream signalling. For example, the CLV complex has been found to be associated with Rho/Rac small GTPase-related proteins; these proteins may act as an intermediate between the CLV complex and a mitogen-activated protein kinase, involved in signalling cascades. KAPP is a kinase-associated protein phosphatase, shown to interact with CLV1. KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it. Another important gene in plant meristem maintenance is WUSCHEL, a target of CLV signaling in addition to positively regulating CLV, thus forming a feedback loop. WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the differentiation of the stem c
In vascular plants, phloem is the living tissue that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to parts of the plant where needed. This transport process is called translocation. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλοιός meaning "bark"; the term was introduced by Nägeli in 1858. Phloem tissue consists of conducting cells called sieve elements, parenchyma cells, including both specialized companion cells or albuminous cells and unspecialized cells and supportive cells, such as fibres and sclereids. Sieve elements are the type of cell that are responsible for transporting sugars throughout the plant. At maturity they lack a nucleus and have few organelles, so they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells do contain vacuoles and other organelles, such as ribosomes, before they mature, but these migrate to the cell wall and dissolve at maturity.
One of the few organelles they do contain at maturity is the rough endoplasmic reticulum, which can be found at the plasma membrane nearby the plasmodesmata that connect them to their companion or albuminous cells. All sieve cells have groups of pores at their ends that grow from modified and enlarged plasmodesmata, called sieve areas; the pores are reinforced by platelets of a polysaccharide called callose. They are of two types and chlorenchyma. Other parenchyma cells within the phloem are undifferentiated and used for food storage; the metabolic functioning of sieve-tube members depends on a close association with the companion cells, a specialized form of parenchyma cell. All of the cellular functions of a sieve-tube element are carried out by the companion cell, a typical nucleate plant cell except the companion cell has a larger number of ribosomes and mitochondria; the dense cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata. The common sidewall shared by a sieve tube element and a companion cell has large numbers of plasmodesmata.
There are two types of companion cells. Ordinary companion cells, which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube. Transfer cells, which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer, they are specialized in scavenging solutes from those in the cell walls that are pumped requiring energy. Albuminous cells have a similar role to companion cells, but are associated with sieve cells only and are hence found only in seedless vascular plants and gymnosperms. Although its primary function is transport of sugars, phloem may contain cells that have a mechanical support function; these fall into two categories: fibres and sclereids. Both cell types are therefore dead at maturity; the secondary cell wall increases their tensile strength. Bast fibres are the long, narrow supportive cells that provide tension strength without limiting flexibility, they are found in xylem, are the main component of many textiles such as paper and cotton.
Sclereids are irregularly shaped cells that add compression strength but may reduce flexibility to some extent. They serve as anti-herbivory structures, as their irregular shape and hardness will increase wear on teeth as the herbivores chews. For example, they are responsible for the gritty texture in pears, in winter bears Unlike xylem, the phloem is composed of still-living cells that transport sap; the sap is a water-based solution, but rich in sugars made by photosynthesis. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as tubers or bulbs. During the plant's growth period during the spring, storage organs such as the roots are sugar sources, the plant's many growing areas are sugar sinks; the movement in phloem is multidirectional. After the growth period, when the meristems are dormant, the leaves are sources, storage organs are sinks. Developing seed-bearing organs are always sinks; because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.
While movement of water and minerals through the xylem is driven by negative pressures most of the time, movement through the phloem is driven by positive hydrostatic pressures. This process is termed translocation, is accomplished by a process called phloem loading and unloading. Phloem sap is thought to play a role in sending informational signals throughout vascular plants. "Loading and unloading patterns are determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata."Organic molecules such as sugars, amino acids, certain hormones, messenger RNAs are transported in the phloem through sieve tube elements. Because phloem tubes are located outside the xylem in most plants, a tree or other plant can be killed by stripping away the bark in a ring on the trunk or stem.
With the phloem destroyed, nutrients cannot reach the